1. Field of the Invention
This invention relates to a split-flow-type flow sensor apparatus including a main flow passage and a bypass flow passage with a thermal flow sensor placed on the wall face of the bypass flow passage.
2. Description of the Related Art
Flow sensors for measuring the flow quantity of gas include a sensor called a thermal flow sensor. With the sensor, a resistor such as a platinum wire is exposed to a gas flow and the degree of cooling heat produced when an electric current is allowed to flow into the resistor by the gas flow is detected, whereby the gas flow quantity is measured.
In recent years, a small thermal flow sensor formed with a platinum thin film on a silicon chip several millimeters square using a processing technique called MEMS (Micro Electro Mechanical System) has been developed for use with the development of semiconductor manufacturing technology. A flow sensor apparatus wherein such a thermal flow sensor is attached to the inner wall of a flow passage and is integral with a squeezing section and a rectification section is commercially practical. (For example, refer to JP-B-6-43907) The flow sensor apparatus is inserted into a midpoint in piping of gas in an apparatus or a facility wherein the flow quantity is to be measured for use.
In the flow sensor apparatus using the small thermal flow sensor as described above, if the flow quantity of gas increases, the gas flow in the flow passage tends to be disordered and the measurement accuracy tends to be degraded. Then, the gas flow passage is separated into a main flow passage and a bypass flow passage and a thermal flow sensor is placed on the wall face of the bypass flow passage for estimating the whole flow quantity from the partial quantity of flow through the bypass flow passage. This means that the whole flow quantity is estimated from the separation ratio between the main flow passage and the bypass flow passage. If the separation ratio is changed, the range of flow quantity that can be measured can be changed.
A flow sensor apparatus having a main flow passage and a bypass flow passage in one piece is also developed. (For example, refer to JP-A-2003-329504) Such a flow sensor apparatus including a main flow passage and a bypass flow passage and having a thermal flow sensor placed on the wall face of the bypass flow passage is called a split-flow-type flow sensor apparatus. In the flow sensor apparatus disclosed in patent document 2, the bypass flow passage is called sensor flow passage and the main flow passage is called bypass flow passage.
To enhance the measurement accuracy of the split-flow-type flow sensor apparatus as mentioned above and provide stable measurement output, it is important to smooth the flow of gas in the main flow passage and the bypass flow passage for minimizing disorder of the gas flow. Particularly, it is important to minimize disorder of the gas flow on the periphery of the thermal flow sensor in the bypass passage. On the other hand, it is also necessary to meet the demands for miniaturizing the split-flow-type flow sensor apparatus and reducing the cost thereof.